1. Field of the Invention
The invention relates generally to microwave switches and modulators. More particularly, the invention relates to a microwave switch/modulator which is controlled by optical illumination from a laser light source or the like. The illumination causes variations in the reactance of the switch/modulator photodiode, thereby detuning a resonant circuit, causing RF signals to be reflected rather than absorbed or transmitted. The microwave switch/modulator is also electronically tunable so that the microwave frequency of operation can be varied by adjusting the reverse bias voltage to the photodiode.
2. Description of Related Art
High-speed RF and microwave switches are used in numerous applications. They are often used in radar receivers as blanking switches to protect sensitive circuitry from damage during the transmit pulse. Fast switching is necessary for this application in order to minimize the period of time in which the receiver is disabled after the transmit pulse has ended so the radar can detect close targets.
Heretofore, high-speed RF and microwave switches have employed optoelectronic microwave switching technology, which relies on photoconduction to achieve microwave switching. Photoconductivity is an increase in conductivity with illumination. Thus prior art optoelectronic switches rely on a change in the device resistance to effect the switching function.
Prior art optoelectronic microwave switches can be classified according to the means of operation. Plasma switches work by forming a highly conductive plasma in the electrical path of a microwave transmission line. The plasma-forming semiconductor material can be placed as a shunt element, a series element or both. See Platte "Optoelectronic Microwave Switching", IEE Proceedings, Volume 132, No. 2. pp 126-132, April, 1985.
It has also been proposed to use light of two different wavelengths to turn on and off a plasma switch constructed as a simple gap in a microstrip transmission line on a semiconductor substrate. One wavelength of light is absorbed in the surface of the semiconductor and forms a conducting plasma across the microstrip gap allowing RF transmission. The other wavelength penetrates deep into the semiconductor, forming a conducting plasma to ground which reflects the incident RF energy. For more information on this technique, see A. M. Johnson, D. H. Auston, "Microwave Switching By Picosecond Photoconductivity", IEEE - QE, Volume QE-11, No. 6, pp 283-287, June, 1975.
Plasma switches turn on very rapidly (10-100 pS) but turn off very slowly (1 uS-1 mS) because there is no electric field present to sweep away carriers. The turnoff time is determined by carrier recombination unless some means of shunting is used as described above. A second disadvantage of the plasma switch is the requirement for a high-powered laser to form a plasma over a large region or surface.
Avalanche photodiode switches have been used as microwave switches because of their high photoconductivity. The diode is typically biased near avalanche breakdown, where a small amount of illumination causes ionization in the high field region. The electron-hole pairs in turn ionize other atoms creating large photocurrents through multiplication. For more information on this effect, refer to R. A. Kiehl, "An Avalanching Optoelectronic Microwave Switch", IEEE - MTT, Volume MTT 27, No. 5, pp 533-539, May, 1979. Avalanche photodiodes switch comparatively slowly because the avalanche process takes time to build. Also, they are noisy and can have thermal problems.
Bias controlled optoelectronic switches represent a whole class of switches which use bias control to turn optical detectors on and off. For instance, a photodiode can switch on and off the detected microwave modulated illumination from a laser by transitioning from reverse to forward bias, respectively. In other words, photodiode detectors only function in reverse bias. When they are switched to forward bias, they will not detect amplitude modulated microwave energy on an optical carrier. In a similar way, an avalanche photodiode can be turned on and off by switching the bias in and out of avalanche. Bias controlled optoelectronic switches thus rely on changes in bias in order to switch. As a result, this type of switch nas the same speed as a conventional PIN diode switch (on the order of 10 nS).